Although it is not primarily a special effects picture, much credit has to go to photographic effects supervisor Douglas Trumbull, who achieves some really magnificent imagery here. The spaceship is an incredible piece of artistry and design, and Trumbull's effects inspire the requisite amount of awe and wonder, without which the film would fall apart.
It is Spielberg's emphasis on the human condition that make the biggest impression, however. The scenes of Dreyfuss' mundane home life reveals the banality of the existence he has been living, making his character's curiosity about "what's out there" all the more profound. Spielberg does a remarkable job capturing that sense of wonder that keeps people watching the skies. Indeed, the film's rather protracted conclusion reveals perhaps too much detail, removing some of the mystery that comes from things left unseen. Perhaps that's why the ending -- in which the alien beings and the fate of their abductees are revealed -- seems a bit anticlimactic. As well-done as it undoubtedly is, it's simply too literal, and thus a bit of a let-down from the preceding events of the film.
However, the ending does force the viewer to ask themselves if they would leave behind their family, their home, and indeed, their very beliefs, in exchange for the experience of making contact with extraterrestrials and traveling with them to places unknown -- questions we may find ourselves pondering on clear nights under the vast expanse of shining stars, and wondering what's out there.